An FTIR is used for qualitative analysis and quantitative analysis of substances and can be employed for a broad range of substances, whether organic or inorganic. In an FTIR, analysis is performed by moving a movable mirror back and forth and acquiring an interference signal, but since the detector has a frequency characteristic, when collecting data, it is important for the movable mirror to move at a constant velocity. To achieve constant velocity, feedback control is performed, whereby the current velocity is acquired and the error between it and the target velocity is corrected. A laser interferometer is employed for acquisition of the current velocity. When the position of the movable mirror changes, the intensity of the laser interference signal will change, and if the movable mirror is moving at a constant velocity, the laser interference signal will be detected as a sine wave of constant frequency. If this interference signal is treated as a signal having an amplitude in the plus direction and minus direction from ground level (known as a fringe signal), the time during which the movable mirror has moved the distance of the optical path difference corresponding to one wavelength of the laser can be measured by measuring the time of the interval from one rising zero crossing (the time point of rising from ground level) to the next rising zero crossing. Namely, the velocity of the movable mirror can be measured.
The present inventor has proposed an FTIR which comprises a movable mirror control device comprising a control interferometer of a type wherein a movable mirror is suspended such that its initial balance position is in the gravity direction, and which, using measured values of the current position and current velocity of the movable mirror, performs feedback control of the current provided for driving the movable mirror such that the movement velocity of the movable mirror will be at the target velocity, based on values obtained by applying gain to the distance from the balance position of the movable mirror, the current velocity of the movable mirror and the difference between the current velocity and the target velocity of the movable mirror (see Patent Literature 1). Here, taking at least a portion of these gains as variable parameters, adjustment is performed such that the variable parameters will be at an optimal value in order to bring the movement velocity of the movable mirror to the target velocity at the time of adjustment.
As the object, the present invention specifies not this sort of movable mirror but rather an object which moves in a straight line along a linear guide. This object may be the movable mirror itself, or another component. In the case where it is another component, the movable mirror is integrally mounted thereon. In the case of a movable mirror which moves in a straight line along a linear guide, voltage is applied to a coil which drives the movement of the movable mirror, and the movable mirror moves on the linear guide due to electromagnetic force generated by current flowing through the coil and a magnetic field created by a magnet, so the velocity of the movable mirror is controlled by successively changing the voltage applied to the coil which drives the movement of the movable mirror.